There are several types of known pipe junctions. An example of such a junction is given in document FR-A-2 196 053, which describes an assembly in which one end of a pipe is fixed to an end section fitted with a shoulder and a hemispherical surface, the end of the second pipe being fitted with an external thread cooperating with a nut serving to tighten the elements together. This screw assembly provides both the sealing and junction functions, which could cause problems, especially in cases where there is a high pressure inside the piping.
According to another type of pipe junction, such as described for example in document FR-A-2 284 079, there is provided a deformation of each of the pipe ends, with a flange formed on the male end and an externally threaded taper for the female end. These ends are joined by a nut after prior interposition of an annular sealing junction. Although this type of junction dissociates the sealing and junction functions, it is not applicable to the type of piping used to convey fluids or gas at high pressure, high temperature or under severe operating conditions, e.g. as regards vibrations.
Other types of screw-type metallic junctions are also known, e.g. through French patent FR-A-2 107 414. In this type of junction, the sealing element is, however, in this case not in direct contact with the inside of the piping, and the connection is relatively complex.
There is also known, e.g. through document U.S. Pat. No. 3,142,498, a lipped junction in which the end of one of the pipes to be assembled has a lip forming a sealing junction. In this case, the lip is formed directly in the terminal portion of the piping, and as a result has an elasticity or mechanical resistance that is reduced on account of the fact that the lip must necessarily be made of the same material as used for the pipe.
Document U.S. Pat. No. 4,597,596 discloses a metallic sleeve made of high quality alloy which is fixed to an end of one of the tubes to be joined, e.g. by welding, and which is itself of one-piece construction. The elasticity of the metallic sleeve forming the junction can be fairly large, but the requirement of fixing the sleeve by a single weld in an area of the piping susceptible to receive vibrations for example, restricts the range of usable materials for forming the junction or else makes the assembly fragile.
It has also been proposed to use a junction having a lip made of high-performance alloy and brought onto the tube element by rolling expansion. However, in this case the connection by rolling expansion severely limits the vibration resistance of the connected assembly.
It thus appears that the different known junctions prove to be unadapted to difficult environments i.e. those in which there is a high degree of vibration, or high temperatures, or again where access is difficult for changing junctions, which in turn imposes a very long lifetime on the connecting elements.